Precipitation Behaviors of Carbides in High Speed Steel during ESR and Heat Treatment

The microstructure and carbides evolution of high-speed steel after electroslag remelting and solution treatment were studied. The thermodynamic precipitation mechanism of carbides in solid phase was discussed and the characteristic parameters of carbides in different processes were also investigated. The results show that there were large lamellar and fibrous Mo2C and a small amount of VC in the ESR ingot. Mo2C are metastable carbides, which can be decomposed into VC and Fe2Mo4C during the solution treatment. The average diameter of the carbides is reduced to 1.28 μm and the space distance is reduced to 3.23 μm after forging and hot rolling, which means carbides are completely spheroidal and dispersed in matrix.

Aging precipitation behavior of σ phase in 22Cr15Ni3.5Cu austenitic stainless steel

σ phase is one of the main precipitates affecting the toughness of austenitic stainless steel, V-notch impact test, SEM, EDS and TEM analysis were conducted on the newly developed 22Cr15Ni3.5Cu stainless steel after 650°C aging. Precipitation mechanism of σ phase and its effect on the toughness of the material were analyzed. The test results show that toughness of the material decreases to 25.6J after 300h aging, σ phase started to precipitate along the grain boundary after 500h aging, and in the crystal after 1000h aging. The precipitation spacing is about 100 nm, forming a gradually increasing size from crystal to grain boundary. As the precipitation time 500h of σ phase was later than the critical aging time of ductile brittle transition, it can be inferred from the test result that σ phase is not the main precipitation phase affecting the toughness of 22Cr15Ni3.5Cu.

Insight into the Role of Cerium (III) Addition to a MgAl-LDH Coating on AA6082

In this work, Ce doped MgAl-LDHs layers have been developed through an in-situ synthesis method on 6082 aluminum surface. The aim was to gain mechanistic insight into the role of Ce(III) as an active corrosion inhibitor embedded in the LDHs layer. The development of the LDH structure was verified by checking the presence of the characteristic XRD peaks, the platelet morphology (evaluated by SEM-EDXS) and the functional groups (by FTIR-ATR analyses). The same techniques were employed to assess the effect of a prolonged immersion time in 0.1 NaCl on the Ce doped MgAl-LDH coatings. Electrochemical impedance spectroscopy (EIS) was employed to monitor the evolution of the electrochemical properties of the coatings during prolonged immersion in saline solutions. The findings suggest a crystallization/dissolution/precipitation mechanism which implies: (i) the formation of crystalline cerium compounds, such as Ce(OH)3, in the LDH structure during the synthesis; (ii) the dissolution upon exposure to the NaCl solution, thus leading to cerium ions release; (iii) the precipitation of amorphous Ce oxides/hydroxides at the cathodic sites when the metal starts to corrode; (iv), the consequent mitigation of the electrochemical activity of the metal and, thus, the reduction of the extent of corrosion.

Morphology of Barite Synthesized by In-Situ Mixing of Na2SO4 and BaCl2 Solutions at 200 °C

Barite is an abundant sulfate mineral in nature. Especially, the variety of morphologies of barite is often driven by the mixing of Ba-bearing hydrothermal fluid and sulfate-bearing seawater around hydrothermal chimneys. In order to better understand the factors affecting the morphology and precipitation mechanism(s) of barite in seafloor hydrothermal systems, we synthesized barite by a new method of in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C while varying Ba concentrations and ratios of Ba2+/SO42−, and at room temperature for comparison. The results show that barite synthesized by in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C forms a variety of morphologies, including rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped morphologies, while room temperature barites display relatively simple, granular, or leaf-like morphologies. Thus, temperature affects barite morphology. Moreover, dendritic barite crystals only occurred at conditions where Ba2+ is in excess of SO42− at the experimental concentrations. The dendritic morphology of barite may be an important typomorphic feature of barite formed in high-temperature fluids directly mixing with excess Ba2+ relative to SO42−.

Vertical Zoning in Hydrothermal U-Ag-Bi-Co-Ni-As Systems: A Case Study from the Annaberg-Buchholz District, Erzgebirge (Germany)

The Annaberg-Buchholz district is a classic occurrence of hydrothermal five-element (U-Ag-Bi-Co-Ni-As) veins in the Erzgebirge (Germany) with an historic production of ~8,700 metric tons (t) Co ore, 496 t U ore, and 26.9 t Ag. Multiple mineralization stages are recognized in polyphase veins hosted by Proterozoic paragneiss. Fluorite-barite-Pb-Zn mineralization occurs across the entire vertical profile of the district, whereas U and five-element stages are restricted to the upper 400 m below surface, coinciding with a graphite-rich gneiss lithology. Here, we present field and petrographic observations, electron probe microanalysis and fluid inclusion data, as well as thermodynamic calculations to characterize five-element and fluorite-barite-Pb-Zn associations, and to constrain the origin of the vertical zoning in the Annaberg-Buchholz district. Microthermometric analyses of fluid inclusions related to the fluorite-barite-Pb-Zn stage yield homogenization temperatures between 78° and 140°C and salinities between 21.9 and 27.7 equiv wt % (NaCl-CaCl2). A correlation of fluid inclusion Na/(Na + Ca) ratios with salinity suggests fluid mixing as a likely precipitation mechanism and relates ore formation tentatively to regional tectonics of the Mesozoic opening of the Atlantic. Thermodynamic calculations indicate that U is more sensitive to reduction than As, predicting that arsenide minerals are precipitated more distally relative to uraninite upon reduction along the fluid-flow path. This implies that the observed vertical zoning is not a primary feature but is the result of hydrothermal remobilization. The observations made in the Annaberg-Buchholz district have general importance to the understanding of U-rich five-element mineralization and may be relevant for exploration targeting in unconformity-related U deposits.

Origin and occurrence of gem-quality, skarn-hosted barite from Jebel Ouichane near Nador in Morocco

Light-blue barite from Jebel Ouichane in Morocco forms blade-like tabular crystals (up to ca. 10 cm) with superb transparency and lustre and represents one of the most spectacular gem-quality worldwide. The barite is hosted by iron-ore-bearing skarns, developed within Jurassic-Cretaceous limestones, and occurs in close spatial association with calcite. The crystals have their cores enriched in Sr and contain abundant monophase (liquid) fluid inclusions of primary and pseudosecondary origin. The barite probably precipitated slowly at a relatively low supersaturation and under the control of a surface reaction precipitation mechanism. However, there were some episodes during its formation with a fast growth rate and the coupled dissolution and recrystallization processes. A combination of fluid inclusion data and stable δ18O value for barite (+ 6.71‰ VSMOW) suggests that low-salinity barite-forming solutions resulted from the mixing of strongly-diluted meteoric waters (enriched in light oxygen isotope) with magmatic-hydrothermal fluids under low-temperature conditions (< 100 °C). Meanwhile, the mineralizing fluids must have been enriched in Ba, Sr, Ca, Mg, and other elements derived from the alteration of carbonate and silicate minerals in sedimentary and igneous rocks. The coupling between sulphur and oxygen isotope data (+ 16.39‰ VCDT and + 6.71‰ VSMOW, respectively) further suggests that barite crystallized in steam-heated environment, where SO42- derived from magmatic-hydrothermal SO2 reacted with sulphates that originate from the oxidation of H2S under near-surface conditions.

The Sb-Au-district Brandholz/Goldkronach (Fichtelgebirge, Germany): mineralogical indications for the evolution of hydrothermal Sb-mineralization.

&lt;p&gt;The Brandholz/Goldkronach district is situated in the southeastern part of Germany in the Bavarian Fichtelgebirge. Previous literature of the mineralogy of the district is rather descriptive and modern geochemical analysis are entirely missing. In this contribution, we combine petrography, bulk rock-geochemical analysis, SEM-MLA as well as EPMA to infer on precipitation mechanism and ore-forming processes. The quartz-polymetallic-sulfide veins are hosted in Ordovician shists, called &amp;#8220;Phycodenschiefer&amp;#8221;, which were intruded by upper Devonian meta-basalts. Antimony-sulfides are the main ore mineralization inside of the quartz-veins, accompanied by minor auriferous arsenopyrite and pyrite. Petrographic observations suggest a precipitation of an early stibnite phase (stage I). Sb-Pb-sulfides/sulfosalts (stage II) precipitated in fractures and fissures of stage I stibnite with a slightly change to Pb-rich Sb-phases. The antimony-mineralization event evolved from stibnite (Sb&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;3&lt;/sub&gt;), over f&amp;#252;l&amp;#246;ppite (Pb&lt;sub&gt;3&lt;/sub&gt;Sb&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;15&lt;/sub&gt;), zinkenite (Pb&lt;sub&gt;9&lt;/sub&gt;Sb&lt;sub&gt;22&lt;/sub&gt;S&lt;sub&gt;42&lt;/sub&gt;), plagionite (Pb&lt;sub&gt;5&lt;/sub&gt;Sb&lt;sub&gt;8&lt;/sub&gt;S&lt;sub&gt;17&lt;/sub&gt;) to boulangerite (Pb&lt;sub&gt;5&lt;/sub&gt;Sb&lt;sub&gt;4&lt;/sub&gt;S&lt;sub&gt;11&lt;/sub&gt;). Chemical analyses corroborate the petrographic observations and indicate a change in the hydrothermal environment from a Sb- to Sb-Pb dominated system with a distinct geochemical change from Pb-free to Pb-containing Sb-phases. A characterization of the precipitation sequence can be used to improve the understanding of the hydrothermal evolution of the whole Sb-Au-ore system in Goldkronach.&lt;/p&gt;